2. PHP Class Inheritance

person Jason Huangfolder_openCode, PHP, Tutoriallocal_offer, , , access_time September 24, 2009

This simple tutorial is for showing how class inheritance works in PHP and also provide a simple Object Oriented Programming (OOP) concept.

To put things simply, class inheritance means a class can be inherited by a child class. A child class would inherit all the variables & functions the parent class has. Plus a child class can have its own specific variables & its own functions, it can overwrite the parent class’ function as well.

For example, you can have a parent class called “PEOPLE”, then you can have child class that is inherited called “MAN”, or “WOMAN”. Since man & woman is both type of people.

In the following simple example, I define a child class inherit from class “contact_info” in the previous tutorial. The child class called “business_contact_info” inherits all the variables & functions from contact_info, it also has a variable “company” defined which is only unique to child class “business_contact_info”, this child class overwrites the print_contact_info() function (now prints also the company name).

In the example, I also show how to call the parent class’ functions in a child class. For example,  I overwrite the constructor function of class “contact_info” with specific constructor for the child class “business_contact_info”. In the “business_contact_info” constructor function, I call the constructor function of “contact_info” to assign the $name variable.

To call a function defined in parent class. Simply use the syntax [class_name]::[function_name] ([parameters]).

The file: “business_contact_info.php“:

//need to include parent class to define the parent class
include "contact_info.php";
//To inherit from a class, simply add extends <class name>
/* a child class would automatically inherits all the class variables &
   class functions defined in the parent class. Therefore, business_contact_info has
   all the class variables & functions in the parent class contact_info   */
class business_contact_info extends contact_info
	//additional class variables only in this class
	var $company;
	var $title;
	/*Ovewriting the constructor function 
	To set a title for when creating a new business_contact
	public function business_contact_info($name, $title)
		/* name is pass along over to the parent class contact_class
		then the title is set in the business_contact_info class. 
		To call a parent class's function, just use:
		[Parent ClassName]::[Parent class Function Name] (whatever parameter)
		$this->title = $title;
	public function set_company($company)
		$this->company = $company;
	/* To prevent a child class from overwriting a parent class's function
	simply put "final" in the parent's function definition
	For example: to prevent overwrite of function print_contact_info()
	in parent class contact_info. define the function in parent class contact_info as:
	final function print_contact_info() ...
	/* overwriting a function */
	/* in PHP, to overwrite a function that is defined in parent class. 
	simply define a function with the same name in the child class. */	
	public function print_contact_info()
		$output = "Name is: ".  $this->name ."<BR>". 
			       "Phone Number is: ". $this->phone_number. "<BR>". 
				   "Company name: ".$this->company. "<BR>".
				   "Title: ".$this->title. "<BR>".
				   "Note: ".$this->note;
		return $output;
} //end of class

Here is the parent class contact_info.php:

class contact_info
  /* declare some class properties as variables */
  var $name;
  var $phone_number;
  var $note;
  /* Constructor is simply a function with same name as the class */
  public function contact_info($name)
	$this-&gt;name = $name;
  /* To assign a value to the class variable - use $this-&gt;
  /* NOTE: the variablename when use in $this-&gt;, does NOT use the $ in front of variable name */
  public function set_phone_number($phone_number)
	$this-&gt;phone_number = $phone_number;
  public function set_note($note)
	$this-&gt;note = $note;
  /* function print out the contact detail information by appending the value of class variables */
  public function print_contact_info()
	$output = "Name is: ".  $this-&gt;name ."
". "Phone Number is: ". $this-&gt;phone_number. "
". "Note: ".$this-&gt;note;
	return $output;
} //end of class

The file “test_business_contact_info.php” shows how to use the inherited class:

include "business_contact_info.php";
$my_biz_contact = new business_contact_info("Jason", "Software Engineer");
$my_biz_contact->set_note("Jason is a great coder and designer.");
echo $my_biz_contact->print_contact_info();

The output of the code:

Name is: Jason
Phone Number is: 888-999-1111
Company: MonkeyCanCode.com
Title: Software Engineer
Note: Jason is a great coder and designer.

Download all the source code files here: inherit_class_example


  1. Interesting, I`ll quote it on my site later.

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